Determine which retinoids are highly effective in inhibiting N-methl-N-Nitrosourea (MNU) induced mammary carcinogenesis. Determine the time interval that may elapse either before or after the carcinogenic insult without impairing the efficacy of the retinoid as regards mammary carcinogenesis. Determine the efficacy of retinoids relative to hormone dependency of the tumors. The efficacy of the retinoid in inhibiting mammary carcinogenesis will be assessed by evaluating the following parameters of mammary carcinogenesis: percent incidence mammary cancer and benign mammary tumors; number of mammary cancers and benign tumors; number of invasive tumors; number of mitotic figures; and latent period of tumor appearance.